As an increasing demand of the energy saving lighting in the current society, high intensity discharge (HID) lamps have replaced halogen lampa and high-tension mercury lamps and become a new efficient light source in the world due to their advantages such as energy saving and high light, etc. While a ballast in the HID lamp is the most important part that determines the quality of the HID lamp.
Generally, the HID ballasts include HID electronic ballasts and HID inductive ballasts. And the HID ballasts has replaced the HID inductive ballasts due to their outstanding advantages, such as a constant power, less power pollution, high utilization of power, and efficient photoelectric conversion.
FIG. 1 illustrates a schematic of a conventional three-stage conversion HID electronic ballast, which includes a rectifier filter circuit 1, a boost circuit 12, a buck circuit 13, and a full-bridge drive circuit 14.
An input terminal of the rectifier filter circuit 11 is connected with the VC supply voltage, and an output terminal of the rectifier filter circuit 11 is connected with an input terminal of the boost circuit 12. The boost circuit 12 has a control terminal connected with an IC 15, and the output terminal of the boost circuit 12 is connected with an input terminal of the buck circuit 13. A control terminal of the buck circuit 13 is connected with an output control terminal P1 of a MUC (Micro Control Unit) and auxiliary circuit 16, and an output terminal of the buck circuit 13 is connected with an input terminal of the full-bridge drive circuit 14. The full-bridge drive circuit 14 has a first control terminal connected with an output control terminal P2 of a MUC and auxiliary circuit 16, a second control terminal connected with an output control terminal P3 of a MUC and auxiliary circuit 16, a third control terminal connected with an output control terminal P4 of a MUC and auxiliary circuit 16, a fourth control terminal connected with an output control terminal P5 of a MUC and auxiliary circuit 16, and an output terminal connected with a load—a HID lamp.
The rectifier filter circuit 11 includes a rectifier bridge 111 and a capacitor C1. An input terminal of the rectifier bridge 111 is the input terminal of the rectifier filter circuit 11; an output terminal of the rectifier bridge 111 is grounded via the capacitor C1. And the output terminals of the rectifier bridge 111 and the capacitor C1 constitutes the output terminal of the rectifier filter circuit 11.
The boost circuit 12 includes an inductor L1, a diode D1 and a switching transistor Q1. One terminal of the inductor L1 is the input terminal of the boost circuit 12, the other terminal of the inductor L1 is coupled with an anode of the diode D1, and a cathode of the diode D1 is the output terminal of the boost circuit 12. The switching transistor Q1 has a drain electrode coupled with the anode of the diode D1, a source electrode grounded, and a gate electrode being the control terminal of the boost circuit 12.
The buck circuit 13 includes a capacitor C2, a switching transistor Q2, and a diode D2. An anode of the capacitor C2 is the input terminal of the buck circuit 13, and a cathode of the capacitor C3 is grounded. The switching transistor Q2 has a drain electrode coupled with the anode of the capacitor C2, and a source electrode coupled with a cathode of the diode D2. An anode of the diode D2 is grounded. A connection terminal of the switching transistor Q2 and the diode D2 is the output terminal of the buck circuit 13, and a control terminal of the switching transistor Q2 is the control terminal of the buck circuit 13.
The full-bridge drive circuit 14 includes inductors L2, L3, capacitors C3, C4, and switching transistors Q3, Q4, Q5, Q6. One terminal of the inductor L2 is the input terminal of the full-bridge drive circuit 14, and the other terminal of the inductor L2 is grounded via the capacitor C3. A connection terminal of the inductor L2 and the capacitor C3 is coupled with a drain electrode of the switching transistor Q3, a gate electrode of the switching transistor Q3 is the first control terminal of the full-bridge drive circuit 14, and a source electrode of the switching transistor Q3 is connected with a drain electrode of the switching transistor Q4. The switching transistor Q4 further has a gate electrode that it is the second control terminal of the full-bridge circuit drive circuit 14, and a source electrode grounded. Concretely, the switching transistor Q5 has a drain electrode connected with the drain electrode of the switching transistor Q3, a gate electrode that is the third control terminal of the full-bridge drive circuit 14, and a source electrode connected with a drain electrode of the switching transistor Q6. A gate electrode of the switching transistor Q6 is the fourth control terminal of the full-bridge drive circuit 14, and a source electrode of the switching transistor Q6 is grounded. A connection terminal of the switching transistors Q3, Q4 is connected with one terminal of the inductor L3, and the other terminal of the inductor L3 is the output terminal of the full-bridge drive circuit 14. And a connection terminal of the switching transistors Q5, Q6 is grounded via the capacitor C4.
The electronic ballast applies low-frequency pulse excitation to light the lamp, and the electronic ballast circuit includes three-stage conversion as following.
Boost conversion. Concretely, the AC is rectified by the rectifier bridge 111 and filtered by the capacitor C1, and then carried out active power factor compensation (APFC) by the IC 15 to eliminate the reactive power. Meanwhile, voltage is increased as the power supply is connected in series with the energy-storage inductor L1, and then rectified and filtered by diode D1 and the capacitor C2, finally is converted into a stable DC voltage of 400V. Thereby, the boost circuit 12 accomplishes the boost conversion.
Buck conversion. As a sequel, the DC voltage of 400V is then discharged via the capacitor C2 and decreased to about 80-120V suited for a full-bridge operating voltage by the switching transistor Q2 controlled by the MUC and auxiliary circuit 16, thereby achieving a constant power operation, meanwhile, the diode D2 is used for clamping. Thereby, the buck circuit 13 accomplishes the buck conversion.
dc-ac conversion. under the control of the muc and auxiliary circuit 16, the full-bridge drive circuit 14 consisted by the inductor 12, capacitor c3, switching transistors q3, q4, q5 and q6, inductor 13, and capacitor c4 coverts the full-bridge operating voltage 80-120 v (dc) into low-frequency square wave pulses with an operating frequency that is lower than 400 hz, and generally between 120-180 hz.
Experimental statistic shows that, “acoustic resonance” may often happen under the operating frequency between 10 KHz to 150 KHz, and rarely happen under the operation frequency higher than 250 KHz. The three-stage conversion HID electronic ballast can efficiently solve the acoustic resonance and constant power problems, but its efficiency will be decreased after any conversion since three conventions are performed. Furthermore, as the magnitude of its operating frequency and the power frequency is the same, thus stroboscopic effect still exists. And a lot of higher harmonics may be generated during the square wave pulses supplying power, which causes electro magnetic compatibility (EMC) test failed.